The development of kidney transplantation has made great strides in the therapy of end stage renal disease (ESRD). It has provided better quality of life (1) and improved survival of patients with ESRD compared with the outcomes of patients on dialysis treatment (2). The shortage of cadaver kidney donors and simultaneously increased demand for kidney transplantation has recently led to a rise in living-donor kidney donation (3,4). However, data on the consequences and its impact on the quality of life and health of living-donors are scarce. The aim of the present study is to examine the long-term changes and function of the remnant kidney after unilateral nephrectomy in an animal model. Here we present our preliminary data on serum creatinine, sodium and potassium levels, and haemoglobin content in red blood cells after the first 6 months of nephrectomy. 36 female Sprague-Dawley rats were randomized into three groups: unilaterally nephrectomized (A), sham-operated (B) and naïve rats (C). In nephrectomized rats (anesthetized by an i.p. injection of 75 mg/kg of ketamin (Ketanest S 25 mg/ml, Pfizer) and 0.5 mg/kg of midazolam (Midazolam Torrex 5 mg/ml, Torrex Chiesi Pharma)) a small lumbar incision was made, and the left kidney was removed. In sham-operated animals, the left kidney was exposed and gently manipulated but left intact. The blood was taken by incision of the tail vein at the inclusion in the protocol and after six months. The data were analysed by Wilcoxon's or Kruskall-Walis test (SigmaPlot 11.0) and considered significant if p<0.05. Initial serum creatinine concentration did not vary among any of the groups (p=0.935). There is a significant increase in serum creatinine concentrations after 6 months in unilaterally nephrectomized rats (39.7±0.8 μmol/l, n=9) compared with sham-operated (30.1±1.1 μmol/l, n=9) and naïve rats (26.3±3 μmol/l, n=5, p≤0.001) (Fig. 1). The serum sodium levels remained the same (p=0.116). Blood haemoglobin content did not differ among the three groups of rats (p=0.115). However, 6 months after the introduction to the study serum potassium level was significantly decreased in sham-operated rats (p=0.006). Although it has been very well established that a kidney possesses a huge capacity to compensate for severe loss of renal mass (5), our results implicate that these reserves can decline with aging. However, reduced function of the remaining kidney found 6 months after nephrectomy does not affect sodium, potassium or haemoglobin levels to any significant extent. In everyday clinical practice we do not see severe renal dysfunction in patients with a single kidney, but in the future prolonged life span and use of more specific biomarkers could support our findings.